An ultrasonic diagnostic apparatus is for imaging diagnostic images based on the echo signals and the reflected signals thereof outputted from an ultrasonic probe. In an ultrasonic probe, a plurality of ultrasonic transducers is disposed. The ultrasonic transducers convert driving signals into ultrasonic waves and transmit them to the object, as well as receive the reflected echo signals produced from the object and convert them into electric signals.
In recent years, ultrasonic probes using a cMUT (Capacitive Micromachined Ultrasonic Transducer) have been developed. The cMUT is an ultrafine capacitance ultrasonic transducer manufactured by semiconductor microfabrication process as disclosed, for example, in Patent document 1. In the cMUT, bias voltage is applied on two electrodes (the object side and the backing layer side) disposed in a plurality of pairs opposite each other in a direction parallel to the ultrasonic wave transmitting/receiving surface, driving signals are superimposed and applied, and ultrasonic waves are sent out.    Patent Document 1: U.S. Pat. No. 5,894,452    Patent Document 2: JP-A-2007-235795